Basic Physical Geography
Mr. Naumann
The earth is a dynamic reality, constantly changing. On the surface of the earth, one
can see the changes caused over time by interior forces operating on the crust of the
earth and exterior forces operating on the surface features of the earth. The interior of
the earth is not a solid mass of rock and produces forces which change the surface
forms and their locations on the earth’s surface. The exterior forces of moving water,
air, and ice work upon the surface features to alter the landscape created by the interior
forces.
50 million
years ago
INTERIOR FORCES
Earth scientists have proven
that the shape and locations of
the continental landmasses
CONTINENTAL
have been and are moving.
DRIFT
Once they were joined together in
one huge landmass which broke
apart. The separate pieces of that original
mega-continent began moving apart until
they came to their present, temporary
configuration on the face of the earth. The
continental drift theory states that the
Today
lithosphere, or crust, of the earth is composed of
many segments or lithospheric plates which are
underlain by less than solid rock matter which is slowly,
but continually, moving. This causes the plates to move
also. Where plates are being caused to move apart, the
gap is filled with rock material which moves up from
below the earth’s crust. The ridge which generally runs
north to south in the middle of the Atlantic Ocean is an
example of this phenomenon. This process causes the distances between Europe and
North America and the
distances between South
America and Africa to
increase as the continents
are pushed, or drift, farther
apart each year. The
process operates very
slowly, because in a period
or fifty to one hundred
years, the changes are not
easily observable. Easily
observable or not, the
movement and change has
occurred and is currently
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occurring. In other places, the plates may be moving toward one another, or one may
be moving toward another. When this happens, one plate usually overrides the other.
The one which is overridden is submerged and its rock material heats and softens and
becomes part of the molten material beneath the lithosphere. These places where
plates meet may be areas with higher than average incidences of volcanic and/or
earthquake activity. The North and South American west coast is an area where this
type of movement is taking place. There are also areas of the ocean, particularly in the
Pacific where surface materials are being drawn down into the earth’s interior. This
occurs in the deep trenches which exist there, often along continental margins. The
lithosphere, then, is not the solid, static reality most people assume it to be, except
when they experience an earth tremor, earthquake, or volcanic eruption. In the normal
day-to-day activities of most individuals, they tend to be unaware of the dynamic nature
of the lithosphere. In reality, the lithosphere is constantly changing as plates move; new
material emerges at mid-ocean ridges from the mantle of earth; lithospheric material of
one plate is drawn down beneath an adjacent plate and softens and rejoins the mantle
material; and earthquakes and volcanic eruptions occur where the stresses built up
within the lithosphere from the interaction of the plates provide opportunities for such
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occurrences. The study of plate tectonics attempts to explain the various kinds of
interactions plates may have with one another in much greater detail.
The stresses caused by the plates of the lithosphere moving and colliding are
responsible for many of the features of the earth’s surface. One category of landforming processes is epeirogeny or epeirogenesis. This is often called continent
building. It involves the raising or lowering of very
large segments of the earth’s surface. This happens
with little or no folding of the surface. It might involve
some warping of the surface. An example of this
would be the creation of a large plateau area. Much
of the continent of Africa is composed of large
expanses of plateaus.
Another category of land-forming processes is diastrophism, the tectonic process
which produces dramatic changes in the shape of the earth’s surface. Diastrophism
includes such processes as orogenesis or
orogeny, faulting, and folding. Orogenesis
refers to the formation of linear mountain
chains on continents. Orogenesis is often
referred to as mountain building. In it, rock
strata , or layers, are thrust upwards in folds
to form a range of mountains. The strata
may also be changed by blocks of the crust
being thrust up in somewhat rectangular
blocks.
When folding occurs, the trough, or valley,
formed where the rock strata folds downward is referred to as a syncline. In the reverse
process, where the rock strata folds upward, the ridge, or higher and rounded area is
referred to as an
anticline. Stresses in
the lithosphere which
produce diastrophism
can cause both faulting
and folding in the same
area. The strains of
folding which produce
an anticline greatly weaken the rock as it is stretched by the folding process. This
causes the ridge or upfold to be very susceptible to the forces of weathering and then
the forces of erosion.
When faulting occurs, a large, deep rupture or crack is created in the lithosphere. The
accumulated stresses, kinetic energy, trapped in the lithosphere will eventually cause
some kind of movement along the rupture or fault. One side may be thrust up as in a
fault block mountain or a tilted plateau such as the Arabian Peninsula. There may be
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horizontal movement more than vertical movement, such as
when two of the lithospheric plates are moving in different
directions, as is the case of the San Andreas fault in California.
In the case of lateral movement, a road which crosses the fault
may be offset and the two sides no longer meet where the
road had crossed the fault. When the accumulated pressure in
the lithosphere is released by movement along a fault, the
movement causes earth tremors and earthquakes.
Earthquakes are one of the two tremendously destructive occurrences that are
associated with the fault zones of the world such as the Pacific rim.
Vulcanism is the process which often results in the other type of disaster associated
with fault zones. When weaknesses in the lithosphere exist, such as those associated
with faults. The molten material of the earth’s mantle, which is under pressure, may
succeed in forcing its way into the lithosphere and may even manage to pass through
the lithosphere and erupt out onto the surface of the earth. The most dramatic example
of this process is a volcanic eruption, but that is not
the only way in which movements of molten rock
material causes changes in the earth’s crust. The
molten rock material below the lithosphere and
sometimes trapped within the lithosphere is referred
to as magma, but once it erupts unto the earth’s
surface, its liquidity increases and it is referred to as
lava.
Magma is molten rock which exists beneath the lithosphere. This is rock material which
can move because it is not in the form of a solid. Some earth scientists believe that it
moves in convectional currents which are powered by the heat of the earth’s core.
Sometimes, this molten material moves into the lithosphere, pushing in between the
rock strata where weaknesses allow it
to intrude. Such a land-forming
process which does not involve the
movement of magma out onto the
surface of the earth is called
intrusion. The magma forcing itself
between layers or rock strata pushes
the rock strata above it upward,
causing a bulge in the surface of the
earth. Over a long period of time, this
molten material cools and solidifies
forming some form of igneous rock.
Some of these intrusions fill vertical
cracks in the lithosphere, whereas
other intrusions are horizontal
between the rock strata. Sometimes
the horizontal intrusions are rather
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thin, while at other times they can be somewhat thick, causing quite a bulge on the
earth’s surface. In this latter case, the softer surface rock may erode after a long time,
leaving an exposed dome of igneous rock material which is referred to as a dome
mountain if it is large [see the illustration on page 4]
When magma, under pressure, is able to intrude into the lithosphere and find a way to
reach the surface the
process is referred to as
vulcanism. This occurs in
many mountain ranges and
contributes to the formation
of mountains. This is also
the process which created
most of the islands in the
Pacific Ocean. The
Hawaiian Islands are the
tops of large volcanoes
which formed in the Pacific
basin. There are still active
volcanoes on the Hawaiian
Islands. In the 1990s,
scientists located a new
The Pacific Rim, where tectonic plates meet, is a
island in the Hawaiian Island
zone of frequent earthquakes and volcanic
chain which is still being
eruptions.
constructed by volcanic
activity. This new island
hasn’t yet become large enough to remain above the surface of the Pacific Ocean, but
volcanic activity there is continuing to build it up toward the surface of the ocean. When
the lava from these eruptions eventually cools, it forms some type of igneous rock too.
EXTERIOR FORCES In addition to the interior forces and processes which cause the
earth’s surface to be a dynamic stage upon which the human story is acted out, there
are other forces which operate from outside the lithosphere and act upon it. These
exterior forces aid in creating some very grand landforms and landscapes such as the
Grand Canyon in Colorado, but they are particularly active in creating the less grand
and less imposing landforms and landscapes with which most people interact daily.
Humans tend to be largely unaware of the actions of interior forces in their day-to-day
lives except when those forces cause an earth tremor, earthquake, or volcanic eruption.
Humans may be somewhat more aware of the action of the exterior land-forming
processes even though they often operate slowly and gradually. People may be aware
that a stream is cutting away at their yard as the forces of erosion and deposition
cause its course to shift. They may have to work hard or spend much money in an
effort to stop the natural eroding actions of the stream, so they will be aware of the
exterior forces.
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The forces associated with water, wind and ice work to change the surface of the earth
by interacting with that surface. The process of weathering causes solid rock to be
broken down into smaller particles by chemical and/or mechanical means. Weathering
occurs in many ways, but all of them prepare materials of the earth’s surface for the
exterior forces of erosion and deposition.
Through changes in temperature, and the
resulting expansion and contraction of the
surface rock of landforms, some types of
rock are more likely to crack and crumble
and flake off. Mechanical weathering
produces the smaller pieces of rock that
are separated from the larger rock are
lighter in weight and can be more easily
carried away by moving water, wind, or
moving ice. This process of earth
materials being relocated by moving
water, air or ice is known as erosion.
EXFOLIATION – the layers of rock
Another type of mechanical weathering
follow rounded joint cracks formed by
results from water entering cracks in rocks
the greater expansion of rock at the
when temperatures are cold. When the
surface in comparison with that
water freezes and the temperature drops
underneath. This is an example of
below four degrees Fahrenheit, the ice in
mechanical weathering.
the crack begins to expand (this is contrary
to how ice behaves above that temperature when it contracts as cold things are
“supposed to behave”). While it is expanding, it is acting like a lever and applying
pressure to the sides of the crack. This makes the crack bigger. On the next cold, wet
night this process is repeated until the rock splits into smaller pieces. This is the same
process that causes potholes in the streets of mid-latitude cities in the winter.
Another kind of weathering process is the chemical weathering process. This involves
the weakening of rock structures by chemical reactions occurring which remove some of
the chemical elements or change the chemical compounds in the rock. Any mineral
compound in rock which can
dissolve in water will be
weakened by exposure to
moisture. Caves in limestone
areas are examples of
chemical weathering. Water
passing through rock,
particularly along cracks in the
limestone, over a long period of
Karst topography caused by the collapsing of
time, dissolves some of the
weakened limestone into what had been large
limestone and leaves cavities in
caves caused by chemical weathering.
what was once solid limestone.
Under some circumstances, water becomes a mild acid and then reacts chemically with
any mineral compounds in rock that can react with an acid. This kind of chemical
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weathering has been accelerated by increasing pollution, particularly around large
urban areas, which causes the phenomenon known as acid rain. Rainwater passing
through a thick ground cover of decomposing leaves, particularly pine needles, also
becomes mildly acidic, or mildly alkaline, depending on the chemicals produced by the
decomposing vegetation. These acidic or alkaline solutions then contribute to the
process of chemical weathering.
Erosion can occur once rock material is reduced in size enough that the force of
gravity, moving water, moving air, or moving ice can cause it to move. When gravity
can overcome the inertia of a rock, it will fall from the face of a cliff or tumble down an
incline. This occurrence qualifies as a form of erosion because earth material has been
removed from its original location. When the moving rock material stops moving, it can
be said that deposition has occurred. Erosion and deposition are the beginning and
end of one process. When earth materials are eroded (moved) from one place, they do
not cease to exist. They are carried by the agent of erosion, moving water, moving air,
or moving ice, and left, deposited, somewhere else, so the process ends in deposition.
Erosion and
deposition occur all
along the stream as
banks erode and
gravel bars and sand
bars are formed.
The faster the agent of erosion moves and the larger the volume of it, the larger the
pieces of earth material it can carry and move. As the erosional agent slows down, the
heavier particles begin being deposited on the earth’s surface. When the erosional
agent stops moving, the smallest particles settle to the surface of the earth and
contribute to some form of depositional landform. Gullies and canyons are examples of
erosional landforms; whereas, deltas, alluvial fans, and terminal moraines are examples
of depositional landforms.
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Landforms are composed of a variety of rock. Igneous rock was mentioned in the
section about interior forces. Igneous rock is rock which has been formed by the
solidifying of magma or lava. There are many kinds of igneous rocks, depending on
what chemical compounds were in the molten matter that cooled to form the rock. The
process of deposition can be the beginning of the formation of another family or rock,
sedimentary rock. Earth materials that have been deposited and left undisturbed for a
long time may be solidified by the pressure of overlying materials and become a form of
sedimentary rock. There are a variety of sedimentary rocks, depending on the kinds of
materials which were deposited. The different kinds of sediment result in sandstone,
limestone, etc. There is a third member of the rock family, metamorphic rock. As its
name suggests, metamorphic rock is rock which has been changed. The varieties of
metamorphic rock all started as either a form of sedimentary rock or igneous rock.
When exposed to enough heat and/or pressure, sedimentary rock and igneous rock are
transformed into harder, less permeable, metamorphic rock. The varieties of
metamorphic rock depend on the kinds of rock they were originally – e.g., limestone
becomes marble when it becomes a metamorphic rock.
THE FOUR MAJOR LANDFORMS Landforms are always being created and changed.
This usually happens so slowly that few people notice it. Volcanic eruptions,
earthquakes, and major floods may create changes which are quickly and easily visible.
There are four major types of landforms and thousands of minor landforms which are
created and changed by the forces discussed above. The four major landforms will be
illustrated and described below, but the minor landforms will not be. It might be
fascinating to study about the many minor landforms, or one category of them like
glacial landforms, in an earth science or physical geography textbook. The diagram
below illustrates a cross section of the major landforms to give an impression of relative
elevations and shapes.
Plains These are generally level or gently rolling land surfaces with low elevation.
They usually have little local relief [the difference in elevation between the highest point
and the lowest point in the area]. Plains are often found along the coasts of continents
or islands. Frequently there may be large inland plains at somewhat higher elevations.
The flood plain is a narrow linear plain formed by the erosional action of a river. Where
water is readily available, plains have been very hospitable to human habitation.
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Plateaus Plateaus are large areas of level or gently rolling land which stand above the
elevation of the surrounding areas. Usually there is little local relief on the plateau’s
surface. There is usually a sharp drop-off called an escarpment between the surface of
the plateau and the lower land next to it. Sometimes plateaus are called tablelands.
Plateaus are usually found in dry climates. Plateaus have been formed in humid
regions, but the amount of water available there accelerates the process of erosion and
the plateau eventually becomes a dissected plateau which, to the naked eye looks like a
region of hills. Examination of the rock strata beneath the surface reveals horizontal
layers or rock strata which testify to this region of hill having originally been a plateau.
The Ozark Highlands or Ozark Plateau is an example of a dissected plateau.
Hills These are areas having elevations
greater than 500 feet and local relief of
more than 500 feet but less than 2,000
feet. Most of the land is sloped rather than
level. In hills, the slopes are usually
moderate rather than steep.
MOUNTAINS OR HILLS? Physical
geographers can’t trust place names to
accurately identify landforms. Place
names are given by the people who
settle an area, not by geographers, and
have a tendency to stick, even if they
are technically inaccurate. The Green
Mountains of Vermont have the
elevations and local relief to qualify
them as hills, but the name Green
Mountains has been used and accepted
for so many years that no one will
support a movement to change the
name to the Green Hills of Vermont.
Mountains Mountains are areas with
elevation usually greater than 2,000 feet.
They are characterized by steep slope,
narrow divides, ridges, and peaks. The
local relief is usually more than 2,000 feet.
Little level land exists in most mountains.
Mountains have a major influence on the
climate of their windward and leeward
sides. They also influence transportation
routes through them. While not absolute barriers to the movement of humans and their
products, mountains discourage such movement and make the task more difficult.
LANDFORMS AND HUMAN USE OF THE EARTH
It is impossible to discuss human activity on the earth without discussing landforms –
they are a major part of the “site” of any location.
SITE refers to the
Using the terminology of the five themes of geography,
characteristics of a place
landforms are a major part of the physical place of a
or location which are
location. Physical geography most definitely studies
provided by nature.
landforms, but all forms of human geography must
also consider the landforms of any area being
examined. Landforms may influence climate and soil, and thereby the vegetation, of an
area. These are additional components of physical place. The studies of historical
geographers have shown that human movements have been encouraged or
discouraged by particular landforms. The studies of economic geographers have
shown that certain economic activities are more likely to be practiced in particular types
of landform areas. Landforms do not determine where people will settle, what routes of
travel they will take, or what types of economic activities they will practice; but they do
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influence the decisions. The critical factors in these decisions are cultural: the level of
technology possessed by a people is a major determiner, and the intensity of their
desire also is important. The way in which two cultures live on and use similar landform
areas may have many similarities or may have significant differences due to their
different cultures.
An example of the influence of landforms can be seen on the largest island of Japan,
Honshu. Only about fifteen percent of the land on that island is relatively level. The rest
is mountainous or hilly, with mountains predominating. If one looks at maps of level
land on Honshu and of population density, one discovers that they look very similar –
most people live on the areas of level land. A study of Japanese culture will show that
they use the areas of level land intensively and with great care to maintain its
productivity. They use as little of their level land for non-productive purposes, houses or
gardens, as possible. On the hilly land adjacent to the level land, they have created
terraces to expand the areas where they can practice their intensive agriculture.
An examination of the Midwest of the United States shows that on such a large area of
level land, people have settled in patterns that show no direct relation to landform types.
The cultural imprint on the land as seen from the air is very rectangular, or geometric.
With such large areas of level land, the human features have followed the geometric
survey system instituted in the late 18th century. Roads, farm boundaries, and political
boundaries often follow these geometric survey lines. Sometimes when one of these
boundaries meets a river, the river was used for part of the boundary.
In the Appalachian mountain region, population is not so highly concentrated in the little
areas of level land as on the island of Honshu, because the people had the option of not
settling in the mountains. Since the Appalachian mountains have lowlands to the east,
west, and south of them, people had the choice of going through them to the west, or
going around them to the south to large areas of undeveloped, level land. The
Appalachians remained a relatively forgotten, isolated part of the United States for a
long time because the major forms of transportation followed the passes through the
mountains or went around the southern end of them. The interest of most people was
to get through or around them to land that was more easily developed. In the
Appalachian area, passes like the Hudson-Mohawk valleys and the Cumberland Gap
became important transportation corridors – there was more development near these
areas than was typical of most of Appalachia. Their “situation” was better than most
areas in Appalachia. A study of
SITUATION refers to the qualities an
highways and railroads that cross the
area has relative to other areas – a town
Rocky Mountains and the Sierra
on a major transportation line may grow
Nevada shows that they follow the
and attract people and businesses,
natural passes through the mountains.
whereas an isolated town far from any
Here too, more people desired to pass
major transportation line will probably not
through these mountains than to settle
attract many people or businesses.
in them.
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It is not difficult to pick out a general principle from these examples: mountains, and hills
to a lesser degree, tend to limit settlement choices and channel transportation; whereas,
level lands like plains are less restrictive. Humans have more easy choices on level
land as long as the climate is relatively desirable. Another general principle is that
human decisions are greatly influenced by culture, particularly the technology level of
the culture.
The submergent coastline of the U.S. east coast with
its many estuaries has many naturally good locations
where port cities have developed. The emergent
coastline of the U.S. Gulf of Mexico coast has few good
locations for large ports. For many years, New Orleans
was the “premier port” along the U.S. gulf coast. After
1945, however, the city of Houston was able to develop
into a major port too, in spite of unfavorable natural
conditions. The island bars which develop offshore
along emergent coastlines had prevented large ships
from reaching Galveston Bay. Modern technology
available after 1945, made it possible to open a channel
large enough for large commercial ships from the Gulf of
Mexico to Galveston Bay. The developing oil industry in
Texas and along the Gulf Coast provided the necessary
desire – humans don’t usually undertake large,
expensive projects just because they are able to do so.
Humans put forth the effort and spent the money
because they have a compelling reason – usually
economic or military.
SUBMERGENCE -epeirogeny has caused a
large portion of the
continental landmass to
decrease in elevation,
thereby flooding valleys
and former coastal plains.
EMERGENCE -epeirogeny has caused a
large portion of the
continental landmass to
increase in elevation
extending the coastline
onto land which had been
part of the continental
shelf. This often results in
a coastline fringed with
island bars which inhibit
access to the actual
coastline.
Before World War I, the Germans had developed
several plans for invading France if was should come between them. One was to
march across neutral Belgium’s plains and risk war with Great Britain, who was pledged
by treaty to protect Belgium’s neutrality. Another was to march through neutral
Switzerland, whose neutrality was not protected by treaty. An examination of a physical
map of Europe makes it clear why the plan that called for marching through
mountainous Switzerland was rejected.
Landforms influence more than just transportation route choices. They can influence
climate, soil, and vegetation. It is a general rule that the steeper the slope of land is, the
thinner the soil is. Gravity and the erosive force of water constantly remove weathered
materials from steep lands and deposits them on more level areas at lower elevations.
The hills and mountains have thin, poorly developed soils and support only the growth
of plants that can grow in thin, poorly developed soils. The bottom lands of the wider
valleys downstream receive much of the eroded material from hilly land upstream.
Here, in the bottom lands, there is much parent material from which good soil can
develop. The bottom lands of the valleys supported much richer vegetation than the
hills, and settlers cleared the forests and used this deep, rich soil for productive
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agricultural purposes. The high productivity of the farms on the Mississippi River flood
plain, in the years without floods, attests to the richness of the alluvial soils developed
on thousands of years of deposited sediments. The lack of wheat, corn, or soybean
fields on the slopes of the Ozark hills attests to the unsuitability of that thinner, less
developed soil for agricultural purposes.
Mountains influence climate. When mountain ridges run roughly perpendicular to the
prevailing winds, the side of the mountain the winds ascend, the windward side, usually
receives much precipitation. The other side, the leeward side where the winds
descend, receives little precipitation and has an arid or semi-arid climate. The Sierra
Nevada and Rocky Mountains in the western part of the United States illustrates this, as
do the Andes Mountains in South America. If these mountains were not there or if they
ran in an east-west direction instead of a north-south direction, the climates of North
and South America would be greatly different from what it is. Because temperatures
vary at different elevations,
getting cooler at higher
elevations, climates within
mountain ranges are very
complex and varied. The
degree of exposure to
moisture bearing winds also
influences the climate
variations within mountain
ranges. Due to the varied
conditions, particularly
temperatures, different kinds
of commercial and
subsistence agriculture are practiced at different elevations. This is referred to as
altitudinal zonation and is illustrated above.
Plains are usually seen as being very conducive to human development. They are easy
to cultivate if they receive adequate rainfall and present few problems for the
construction of cities, roads, and highways. Under some circumstances, though, plains
can present humans with problems to overcome if they want to develop that area.
Plains, particularly at low elevations, often have problems with drainage. If an area
receives much precipitation, the soil may not be able to absorb it and a swampy
condition may exist. The early settlers of New Orleans buried their dead above ground
in vaults because the water table in that area is too near the surface. Houses were built
without basements for that reason also. Lands at higher elevations may have less
trouble with water, even if they receive more precipitation, because the water table is
not so near the surface of the land. Hilly and mountainous regions have little trouble
with water logged soil at their higher elevations because there is much runoff. Where
there is much precipitation, and/or little vegetation on the hills to slow the water, runoff
can because severe floods in the valleys and flood plain areas downstream.
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The nature of the bedrock also influences the water table. Where the bedrock is
porous, limestone or sandstone, the groundwater can pass down into the rock. In areas
with limestone bedrock, water may dissolve passages in the rock (caves), and water
can pass through the soil rather quickly. In the Yucatan Peninsula of Mexico, this
limestone condition, in an area with great precipitation, creates an unexpected condition
– the soil can get too dry if rains don’t come frequently enough because the soil doesn’t
hold much moisture for long. Most of the water moves underground through the caves
and sinkholes rather than being held in the soil. Agriculture in this area depends on
crops that need little water. Irrigation could only be practiced if a reliable source of
water could be found. Where the bedrock is impermeable, not porous, water cannot
leave the soil and enter the bedrock. [Bedrock of igneous or metamorphic rock is more
likely to create this condition.] With impermeable bedrock, the groundwater must slowly
move downhill through the soil. An area like this that receives less rainfall than the
Yucatan could have problems with soil that is too wet. Here, a farmer might have to find
ways to drain his land before he can use it profitably.
CONCLUSIONS
In many ways, human use of the earth is influenced by landforms. Any study of human
activity in a particular location must consider the landforms and their influences. Human
assessment of the usability of particular areas with their particular landforms, however,
is highly determined by culture, particularly the level of technology possessed and used
in that culture. Different groups of people, with different cultures having different world
views and different historical experiences, may assess the usability of an area very
differently.
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